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Planetary News: Deep Impact (2008)Deep Impact: A Comet Chaser Turns Planet HunterBy Amir AlexanderJune 6, 2008
Remember Deep Impact? Three years ago that unusual spacecraft sent an impactor slamming into comet Tempel 1, adding celestial fireworks to the 4th of July celebrations. Scientists are still poring over the data they gathered when that massive jet of cometary debris shot into space, but Deep Impact itself has moved on. On October 11, 2010 the spacecraft will encounter another comet, this one named Hartley 2, and will conduct detailed observations, though without the benefit of an impactor this time. In the meantime Deep Impact has been cast in a very different role: Instead of chasing comets as it was designed to do, the spacecraft is now occupied with the observation and study of extrasolar planets. As it happens, Deep Impact's camera, designed for observing a comet at close range, is also well suited for observing planetary transits. These events occur when a planet is so positioned as to pass directly between its home star and an observer, blocking off part of the star's light until it completes its pass. When monitored from afar, the star's luminosity appears to dip suddenly at regular intervals, before returning to its original level. Transit searches for exoplanets look for this pattern in the luminosity of stars, which is a sure sign of the presence of an orbiting planet. Several of the close to 300 known exoplanets were discovered by this method, and several others discovered by other means were confirmed by their transits. Once the Kepler mission reaches space in February 2009, astronomers believe that it will detect a large number of new exoplanets all by searching for the telltale signs of a planetary transit. Unlike Kepler, Deep Impact is not charged with finding new planets, but with studying several known planetary systems with the object of learning more about them. Deep Impact's observation will focus its attentions on exoplanets that are known to transit, and their passage before their star has been detected. Unfortunately for astronomers, however, the characteristic "dip" for these transits is too small, and the stars too dim for the transit to be observed with the desired sensitivity and precision from Earth. Things, however, look very differently from space. Freed from the atmospheric interference that is the bane of Earth-based observations, Deep Impact's camera can register those slight variations in luminosity that cannot be detected from the ground. To take advantage of this, a group of astronomers led by Drake Deming of NASA'a Goddard Space Center is pointing the spacecraft's camera at five transiting exoplanets in succession. The project, scheduled to last from February to August of this year, is called EPOCh, for "Extrasolar Planet Observation and Characterization." Along with DIXI, the "Deep Impact Extended Investigation," which is the name given to the flyby of comet Hartley 2, the combination of Deep Impact's extended missions is known as EPOXI.
The planets selected for Deep Impact's observations are nothing like our Earth, but belong to a type known as "hot Jupiters" – gas giant planets residing very close to their star and completing each orbit in a matter of days. Because of their size and their short period these are the easiest planets to detect, and they make up a large majority of the exoplanets known to date. By studying their transits Deming and his team expect to do more than simply confirm their presence by another method: they hope to learn about the planets' diameter, density, and atmosphere, and about the possible presence of unobserved and much smaller planets orbiting the same star.
Transit observations can provide a very good estimate of a planet's diameter, because the degree of dimming in the star's luminosity is proportional to the planet's size. When combined with the planet's estimated mass, known from the radial velocity observations, scientists can calculate the planet's density and theorize about its composition. Furthermore, transits measured with highly sensitive instruments can also be used to get an idea of the composition of a planet's atmosphere. This is because a transiting planet, after appearing alongside its star, then disappears behind it in what is known as a "secondary transit." The star's light spectrum when the planet is alongside is a combination of the star's spectrum and that of the planet's atmosphere; the spectrum when the planet disappears behind the star is the star's alone. By subtracting the latter from the former astronomers can deduce the difference: the spectrum of the planet's atmosphere by itself, and hence its composition. Most intriguingly, perhaps, accurately measured transits can point to the presence of smaller and otherwise undetectable planets. This is because the gravitational pull of these invisible planets ever so slightly affects the orbit of the gas giant whose transit is being monitored. In particular, the presence of smaller companions can affect the exact timing of the large planet's transit. If, instead of passing before its star at precisely regular intervals the large planet's orbital period varies slightly, this likely indicates presence of another planet. By monitoring the variations in the timing of the large planet's transits astronomers can learn a great deal about the mass and orbit of its invisible companion. The most promising of EPOCH's targets so far, according to Deming, is a Neptune-size planet orbiting the red dwarf star Gliese 436. The planet, according to Deming, moves in a highly eccentric and seemingly unstable orbit. The fact that it maintains this unusual path almost certainly means that there is an additional undetected planet influencing its course. According to Deming, early analysis indicates that this invisible companion could be no larger than Earth, and might reside within Gliese 436's habitable zone.
Finally, Deep Impact has also been observing and photographing one special planet directly – our very own Earth. These pictures are taken not so much for their beauty (though they are stunningly beautiful), but because scientists believe that they may one day help us detect life on other planets. After all, Earth is the only known planet that supports life, and therefore the only one that can exhibit signs of life to a distant observer. What are those telltale signs, and how exactly can one determine whether a distant planet carries life? Theories and studies abound, including some that analyse "moonshine," the Earth's light reflected back to us from the Moon. But only by observing our planet systematically from space will scientists be able to test their theories and known what to look for in the search for life. Deep Impact has already monitored three exoplanets since the mission began in February, and still has two remaining before the end of August. At that time EPOCh will end, and Deep Impact will begin once more to prepare for a mission it was designed for – observing a comet at close range. Did you like this story? Send
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